I have started working a a 3D engine,using software rendering.I was wondering if someone
could help me with a sample code in OpenGL to draw a pixel with a given color on a
Zbuffer.Also the T&L is handeld by me.
I just want to use OpenGL to plot pixels on a Zbuffer in screen coordinates

If you just want to plot pixels, then there is no reason to use OpenGL.
While OpenGL does that a point primitive, you wouldn't want to use it to
draw the entire 3D scene. To get hardware acceleration you're going to want
to use OpenGL to draw triangles (see www.opengl.org for examples). If for
some reason you don't want OpenGL to do the transforms you can check out
the rasonly examples at: